Techniques for measuring ophthalmic refractions for patients who are aphakic or highly miopic are old in the art and have served refractionists for several years. After these refractions are obtained, however, the vertex distance--the distance between the optical center of the eye and the optical center of the posterior trial lens--and the pantoscopic tilt alteration--the angle at which the spectacle temples are moved off the frame front's perpendicular plane--must still be determined. These latter measurements have caused particular difficulties for refractionists with patients who are aphakic or highly miopic.
Thus, as can be seen from standard vertex conversion charts, a change or error of 1 mm. in the vertex distance can alter the prescription by 0.50 diopter, which results in the patient's having needlessly poor vision. Similarly, since the standard refracting plane of refracting glasses or trial frames is perpendicular to the visual axis while in use, the spectacle temples will be tilted from that perpendicular plane and unless the angle of tilt is properly measures, and an unprescribed cylinder will be created in the dispensed glasses.
Prior attempts to measure vertex distance have involved either complex devices such as the Wesseley Keratometer which itself uses another lens, a scale, and sighting pinhole and which must be altered for negative lenses, or simple devices wherein a disc with a stenopaic slit is inserted in the trial frame in place of the trial lens, and a millimeter ruler passed through the slit until it touches the closed eyelid. The complex instruments are relatively hard to use and expensive. The simple, slit type devices present problems of precision since the distance to the optical center of the trial lens and slit disc may differ since the various trial lenses have different amounts of curvature.
Various protractor type devices for measuring pantoscopic tilt have been deviced, but these devices are separate implements and require the refractionist to acquire two different devices to measure both vertex distance and pantoscopic tilt. Thus, it would be desirable to have one device which can both accurately measure both vertex distance and pantoscopic tilt.